Bi-variable damage model for fatigue life prediction of metal components

Based on the theory of continuum damage mechanics,a bi-variable damage mechanics model is developed,which,according to thermodynamics,is accessible to derivation of damage driving force,damage evolution equation and damage evolution criteria. Furthermore,damage evolution equations of time rate are established by the generalized Drucker’s postulate. The damage evolution equation of cycle rate is obtained by integrating the time damage evolution equations,and the fatigue life prediction method for smooth specimens under repeated loading with constant strain amplitude is constructed. Likewise,for notched specimens under the repeated loading with constant strain amplitude,the fatigue life prediction method is obtained on the ground of the theory of conservative integral in damage mechanics. Thus,the material parameters in the damage evolution equation can be obtained by reference to the fatigue test results of standard specimens with stress concentration factor equal to 1,2 and 3.     

复合材料层合板面内渐进损伤分析的CDM模型简

基于连续介质损伤力学,提出了一个预测复合材料层合板面内渐进损伤分析的模型,它包括损伤表征、损伤判定和损伤演化3 部分. 模型能够区分纤维拉伸断裂、纤维压缩断裂、纤维间拉伸损伤和纤维间压缩损伤4 种损伤模式,定义了与4 个损伤模式对应的损伤状态变量,导出了材料主轴系下损伤前后材料本构之间的关系. 损伤起始采用Puck 准则判定,损伤演化由特征长度内应变能释放密度控制. 假定材料服从线性应变软化行为,建立了损伤状态变量关于断裂面上等效应变的渐进损伤演化法则. 模型涵盖了复合材料面内损伤起始、演化直至最终失效的全过程. 完成了含孔[45/0/-45/90]_2S 层合板在拉伸和压缩载荷下失效分析,结果表明该模型能合理进行层合板的强度预测和损伤失效分析.     

复合材料层合板面内渐进损伤分析的CDM模型

基于连续介质损伤力学,提出了一个预测复合材料层合板面内渐进损伤分析的模型,它包括损伤表征、损伤判定和损伤演化3 部分. 模型能够区分纤维拉伸断裂、纤维压缩断裂、纤维间拉伸损伤和纤维间压缩损伤4 种损伤模式,定义了与4 个损伤模式对应的损伤状态变量,导出了材料主轴系下损伤前后材料本构之间的关系. 损伤起始采用Puck 准则判定,损伤演化由特征长度内应变能释放密度控制. 假定材料服从线性应变软化行为,建立了损伤状态变量关于断裂面上等效应变的渐进损伤演化法则. 模型涵盖了复合材料面内损伤起始、演化直至最终失效的全过程. 完成了含孔[45/0/-45/90]_2S 层合板在拉伸和压缩载荷下失效分析,结果表明该模型能合理进行层合板的强度预测和损伤失效分析.      

Modeling of damage in unidirectional ceramic matrix composites and multi-scale experimental validation on third generation SiC/SiC minicomposites

The purpose of this paper is to experimentally validate a 1D probabilistic model of damage evolution in unidirectional SiC/SiC composites. The key point of this approach lies in the identification and validation at both local and macroscopic scales. Thus, in addition to macroscopic tensile tests, the evolution of microscopic damage mechanisms – in the form of matrix cracks and fiber breaks – is experimentally analyzed and quantified through in-situ scanning electron microscope and computed tomography tensile tests. A complete model, including both matrix cracking and fiber breaking, is proposed on the basis of existing modeling tools separately addressing these mechanisms. It is based on matrix and fiber failure probability laws and a stress redistribution assumption in the vicinity of matrix cracks or fiber breaks. The identification of interfacial parameters is conducted to fit the experimental characterization, and shows that conventional assumptions of 1D probabilistic models can adequately describe matrix cracking at both macro- and microscopic scales. However, it is necessary to enrich them to get a proper prediction of ultimate failure and fiber break density for Hi-Nicalon type S fiber-reinforced SiC/SiC minicomposites.     

Simplified model for prediction of dynamic damage and fracture of ductile materials

A simplified model for prediction of dynamic damage and fracture of ductile material has been proposed. The plastic flow of matrix and the void growth are dealt with separately after we show that two separated loading sub-surfaces and corresponding normality rules for matrix and damage exist. The equation of the loading sub-surface for the matrix plastic flow is derived by the means of introducing a mapping damage-free solid of matrix material, whose constitutive relation is supposed to have been determined via Hopkinson bar experiments. Based on the results of recovered experiments the law of damage evolution is phenomenally established. The model has been applied to predict some spall experiments carried out on tantalum, and results show it predicts the experiments very well.     

Damage evolution and energy dissipation of polymers with crazes

Craze damage evolution and energy dissipation of amorphous polymers with crazes have been studied. A mathematical model of a single craze (SC) is proposed by adopting the fibril creep mechanism. The viscoelastic characteristics of craze fibrils are supposed to obey the Maxwell model and the craze fibrils are assumed to be compressible. The assumption of Kausch [H.H. Kausch, The role of network orientation and microstructure in fracture initiation, J. Polym. Sci. C 32 (1971) 1–44] is adopted to describe the rupture of stressed fibril bonds. The craze damage evolution and the energy dissipation equations of a SC are derived. The equations are solved numerically and the life of a SC is computed. In a significant range of far-field stress, the dissipated energy varies linearly with the stress. Using the proposed model, the uniaxial stress-strain relation of polymers with low-density craze arrays (PLDCA) is investigated. The damage evolution equation of PLDCA is derived, which shows the mathematical relation between the damage of a SC and that of PLDCA. Based on the computed results, the variation of life of PLDCA with respect to applied stress is determined. Discussions are then given to the results and some significant conclusions are drawn.     

Analysis on interface damage of fiber reinforced composites under cyclic load

Based on the shear-lag model, the interface damage of fiber reinforced composites near a matrix crack under cyclic load is analyzed. The governing equations for load and unload processes are derived and solved. The effective debond stress and the debond velocity are then obtained. Furthermore, the degradation of friction stress on the interface caused by slipping is taken into account. Results are given for the degradation behavior and the steady state of debonding.     

Micro-mechanical approximation to the stress field around an inclined fiber of FRCC

Matrix spalling or crushing is one of the important mechanisms of fiber-matrix interaction of fiber reinforced cementitious composites (FRCC). The fiber pullout mechanisms have been extensively studied for an aligned fiber but matrix failure is rarely investigated since it is thought not to be a major affect. However, for an inclined fiber, the matrix failure should not be neglected. Due to the complex process of matrix spalling, experimental investigation and analytical study of this mechanism are rarely found in literature. In this paper, it is assumed that the load transfer is concentrated within the short length of the inclined fiber from the exit point towards anchored end and follows the exponential law. The Mindlin formulation is employed to calculate the 3D stress field. The simulation gives much information about this field. The 3D approximation of the stress state around an inclined fiber helps to qualitatively understand the mechanism of matrix failure. Finally, a spalling criterion is proposed by which matrix spalling occurs only when the stress in a certain volume, rather than the stress at a small point, exceeds the material strength. This implies some local stress redistribution after first yield. The stress redistribution results in more energy input and higher load bearing capacity of the matrix. In accordance with this hypothesis, the evolution of matrix spalling is demonstrated. The accurate prediction of matrix spalling needs the careful determination of the parameters in this model. This is the work of further study.     

Revised damage evolution equation for high cycle fatigue life prediction of aluminum alloy LC4 under uniaxial loading

The fatigue life prediction for components is a difficult task since many factors can affect the final fatigue life. Based on the damage evolution equation of Lemaitre and Desmorat, a revised two-scale damage evolution equation for high cycle fatigue is presented according to the experimental data, in which factors such as the stress amplitude and mean stress are taken into account. Then, a method is proposed to obtain the material parameters of the revised equation from the present fatigue experimental data. Finally, with the utilization of the ANSYS parametric design language (APDL) on the ANSYS platform, the coupling effect between the fatigue damage of materials and the stress distribution in structures is taken into account, and the fatigue life of specimens is predicted. The outcome shows that the numerical prediction is in accord with the experimental results, indicating that the revised two-scale damage evolution model can be well applied for the high cycle fatigue life prediction under uniaxial loading.     

纯弯曲梁的应变损伤失效分析和预测

提出了纯弯曲梁的应变损伤失效分析方法,与Kachanov的材料受载横截面减少定义拉伸损伤变量类似,以梁的弯曲惯性矩阵减少定义弯曲损伤变量。推导了梁的弯曲应变损伤基本方程,其中的材料常数可由Kachanov拉伸损伤模型的材料常数确定。并且提出了便于工程应用的应变失效预测方程。     

含冲击损伤高强中模碳纤维复合材料层压板压缩剩余强度分析与试验验证

论文以碳纤维复合材料层压板为研究对象,发展了一种模拟复合材料层压板冲击及冲击后压缩的一体化数值分析方法．基于Puck 失效准则和粘聚区模型描述层内损伤与层间损伤,分别采用基于断裂能的双线性型、函数型以及直接折减型等不同损伤折减方法构建了层内损伤预测与演化模型;建立了碳纤维复合材料冲击后压缩数值仿真模型,通过开展不同能量冲击后压缩试验,验证了所发展的数值分析方法的有效性;研究结果表明,采用Puck 失效准则和基于断裂能的双线性损伤演化模型预测冲击后压缩强度时具有较高精度．     

压电/纤维复合材料耦合旋转驱动器力学设计模型

本文在经典层合板理论和三维压电材料本构方程的基础上,依据压电材料与纤维复合材料之间的相互作用、位移连续条件,建立了压电—纤维复合材料旋转耦合驱动器力学模型,并推导出电耦合方程。计算和初步实验说明,这种新型结构的压电伸缩变形—离轴纤维复合层扭转变形耦合旋转驱动器比同类型驱动器能量密度高、转矩大,且这种旋转电机结构简单紧凑、工作稳定、寿命长、预计随着研究的进一步深入,可望成为一种新型的微致驱动器而广泛应用于微型机械、精密测量和自动控制等领域。     

Kevlar纤维增强复合材料动态压缩力学性能实验研究

通过实验较系统地研究了Kevlar纤维增强复合材料的动态压缩力学性能,实验结果表明,在冲击压缩载荷作用下Kevlar纤维增强复合材料有明显的损伤软化现象和应变率效应,针对Kevlar纤维增强复合材料动态应力应变实验曲线,提出了含损伤的率相关动态本构方程,由于所引入的损伤最反映了Kevlar纤维增强复合材料内部基体开裂、脱层、纤维断裂等多种破坏模式的总体效果,因此所提出的本构方程形式相对说来比较简便并易于嵌入目前有关冲击力学的有限元或有限差分程序,有一定的工程应用价值。     

A nonlinear constitutive model of unidirectional natural fiber reinforced composites considering moisture absorption

Moisture absorption in natural fiber reinforced composites causes remarkable degradation of mechanical properties. A nonlinear constitutive model is proposed to study the effect of the water uptake on the mechanical properties of unidirectional natural fiber reinforced composites. Accompanying the water absorption in the composites, there are several irreversible thermodynamic processes such as fiber degradation and interface damage. The energy dissipation induced by these processes is described by an internal variable, and two degradation parameters representing interface damage and fiber degradation are introduced to reflect the modulus reduction of the composite. Particularly, the model is used to derive the evolution of elastic moduli influenced by the moisture absorption. The predictions from the present model show a good agreement with experiment results of sisal fiber unidirectional reinforced composites.     

复合材料层合板低速冲击响应和损伤分析

通过在有限元软件Abaqus/Explicit中编写用户材料子程序VUMAT,建立了一种基于能量演化的复合材料低速冲击渐进损伤模型．该模型以三维Hashin准则来预测层内损伤的起始,以一种简化的损伤变量来模拟层内损伤的演化,将具有双线性牵引-分离本构的零厚度界面单元插入层间来模拟分层损伤．采用该模型对14.7 J冲击能量下的纤维增强复合材料低速冲击过程进行了仿真分析,计算得到的冲击力-时间曲线、能量-时间曲线和损伤分布与试验结果吻合较好．根据数值模拟结果,分析了纤维、基体和分层损伤的扩展规律,为低速冲击下的复合材料结构设计提供了理论依据．     

基于Ottosen本构关系的混凝土三维损伤模型

提出基于混凝土已有本构模型建立混凝土三维损伤模型的方法，该方法无需再进行新的参数标定，计算损伤不必通过对损伤演化方程的积分获得。基于Ottosen本构模型建立了相应的损伤模型，就损伤参量和Ottosen非线性指标的不同选取方式，对典型比例加载情形的全过程进行损伤分析，发现基于刚度下降和基于变形能力损失的两种不同损伤描述具有互补性，非线性指标采用比例增大法计算较为简单合理。     

改进的Jones—Nelson物理非线性材料模型与橡胶复合材料层板分析

Ｊｏｎｅｓ－Ｎｅｌｓｏｎ模型是复合材料物理非线性应力应变关系的一种描述方法，它通过建立材料刚度与应变能密度的关系以及考虑纤维增强复合材料的物理非线性问题，非线性的材料矩阵只为应变能的子数，使得材料模型可以方便地应用地复杂应力状态下，通过 在大变形的应变应力变量和变泊松比概念，对Ｊｏｎｅｓ－Ｎｅｌｓｏｎ模型进行了改进，解决了材料特性的扩展问题和收敛问题，同时考虑了大变形中纤维铺设角度的重新取向，使其     

The effect of anisotropic damage evolution on the behavior of ductile and brittle matrix composites

Anisotropic damage evolution laws for ductile and brittle materials have been coupled to a micromechanical model for the prediction of the behavior of composite materials. As a result, it is possible to investigate the effect of anisotropic progressive damage on the macroscopic (global) response and the local spatial field distributions of ductile and brittle matrix composites. Two types of thermoinelastic micromechanics analyses have been employed. In the first one, a one-way thermomechanical coupling in the constituents is considered according to which the thermal field affects the mechanical deformations. In the second one, a full thermomechanical coupling exists such that there is a mutual interaction between the mechanical and thermal fields via the energy equations of the constituents. Results are presented that illustrate the effect of anisotropic progressive damage in the ductile and brittle matrix phases on the composite’s behavior by comparisons with the corresponding isotropic damage law and/or by tracking the components of the damage tensor.     

纤维增强树脂基复合材料多轴疲劳研究进展

纤维增强树脂基复合材料结构在工程应用中常常承受复杂载荷的作用, 从而使材料处于多轴循环应力条件下. 为了更加合理地进行复合材料结构设计, 必须对复合材料多轴疲劳行为进行深入的研究, 建立比较理想的复合材料疲劳寿命预测模型. 首先简单回顾了复合材料单轴疲劳损伤判据及其寿命预测模型, 然后结合作者的研究结果, 总结了近年来国内外纤维增强树脂基复合材料多轴疲劳理论的研究成果, 对各种失效准则、疲劳寿命预测模型进行了较为深入的分析, 指出了它们各自的特点及其存在的问题, 并对复合材料多轴疲劳理论的研究趋势作出了展望. 最后, 对目前常用的复合材料多轴疲劳实验方法进行了总结, 并指出了各种方法的优缺点及其实验中存在的困难.      

纤维杆元模型在框架结构非线性分析中的应用

本文基于Fiber模型,结合分段变刚度概念,推导建立了便于框架结构非线性分析应用的纤维杆元模型,即由杆件两端弹塑性区的纤维子单元及杆件中部弹性区的弹性子单元组成。弹塑性区由于受力较大,采用精细的纤维子单元模拟,中间弹性区受力相对较小,直接采用弹性子单元代替;同时,应用增分理论,分别推导了三维纤维子单元及弹性子单元的单刚矩阵,并采用静力凝聚方法,集成了三维纤维杆元模型的刚度矩阵。另外,直接应用混凝土及钢筋的本构关系,利用该计算模型对一栋12层框架结构进行了时程非线性分析,计算结果与振动台试验结果进行了比较,比较结果表明两者在结构动力特性,位移响应方面等均吻合较好,验证了该模型的有效性,可为推广使用提供参考。